The present exemplary embodiments relate to the generation of electric power using liquid crystals. It finds particular application in conjunction with the generation of power via the change of dielectric properties of liquid crystals, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
In recent years, there has been a well documented need for the small-scale generation of electrical power to power personal electronic devices. Various materials and compositions have been used in devices designed to convert between mechanical and electrical energy.
Several “smart materials” have been used to convert between electrical and mechanical energy with limited success. These smart materials include piezoelectric ceramics, shape memory alloys and magnetostrictive materials. However, each smart material has a number of limitations that prevent its broad usage. Certain piezoelectric ceramics, such as lead zirconium titanate (PZT), have been used to convert electrical to mechanical energy. While having suitable efficiency for a few applications, these piezoelectric ceramics are typically limited to a strain below about 1.6 percent and are often not suitable for applications requiring greater strains than this. In addition, the high density of these materials often eliminates them from applications requiring low weight. Irradiated polyvinylidene difluoride (PVDF) is an electroactive polymer reported to have a strain of up to 4 percent when converting from electrical to mechanical energy. Similar to the piezoelectric ceramics, the PVDF is often not suitable for applications requiring strains greater than 4 percent. Shape memory alloys, such as nitinol, are capable of large strains and force outputs. These shape memory alloys have been limited from broad use by unacceptable energy efficiency, poor response time and prohibitive cost.
It is known that if a capacitor is charged, and the capacitance is then changed by pulling the charged plates apart, mechanical work is done and electrical energy is produced. This is the basis of existing ‘heel-strike generators’ using elastomer capacitors. The use of electroactive polymers in the fabrication of devices to convert between mechanical and electrical energy is disclosed in U.S. Pat. No. 6,545,384 to Pelrine et al. and related patents. These patents disclose polymers that convert between electrical and mechanical energy. When a voltage is applied to electrodes contacting a pre-strained polymer, the polymer deflects. This deflection may be used to do mechanical work. Similarly, when the polymer deflects, an electric field is produced in the polymer. This electric field may be used to produce electrical energy.
Liquid crystal cells, such as those used in displays, are also capacitors, with capacitance determined by the thickness and area of the film and by the dielectric permittivity of the liquid crystal material. The dielectric permittivity in turn depends on the liquid crystal orientation; its effective value can be changed by changing the director orientation. The director orientation of the liquid crystal can be changed in several ways. One way to change the director orientation is by mechanical means, which thereby induces a flow of the liquid crystal. This can be accomplished in a number of ways, such as by reducing the separation of the plates or by mechanical shear (moving one plate relative to the other one) or by creating a pressure difference between two edges of the film.
In liquid crystals, there is coupling of the flow and director rotation. Flow reorients the director, and, conversely, director reorientation induces flow. When flow is induced in a liquid crystal, the flow induced director reorientation results in a change of the capacitance of the cell. For example, if the liquid crystal molecules are oriented initially perpendicular to the film surfaces of the liquid crystal cell, shear flow will typically reorient them so that they become almost parallel to the surfaces. If the dielectric constant is larger in the initial configuration then in the final one, as is the case for materials with positive dielectric anisotropy, the capacitance of the cell will decrease with shear.
If the capacitor is initially charged to a voltage Vo, the reorientation due to shear will cause the voltage to rise. Alternately, if a dc voltage is applied to the capacitor, say from a battery, the shear induced reorientation will result in an electric current flowing into the voltage source, charging the battery. When the flow stops, there is no shear and the director relaxes to its original configuration. This enables periodic repetition of the above process.